1. Field of the Invention
The present invention relates to an automobile antenna which is capable of receiving both AM and FM broadcasts (hereafter referred to as "AM waves" and "FM waves"), and more specifically to an automobile antenna with improved reception sensitivity.
2. Prior Art
As shown in FIG. 10, conventional AM/FM automobile antennas are generally constructed as follows: An antenna element 1, which is either extendible or not extendible, is attached to a vehicle body wall 2, and one end of a coaxial feeder cable 4 is connected to a feeding point 3 of the antenna element 1. The other end of the coaxial feeder cable 4 is connected (either directly or via a relay cable) to a receiver set 5 which is installed inside the vehicle.
The length of the coaxial feeder cable 4 is appropriately set in accordance with the physical distance between the antenna feeding point 3 and the receiver set 5 which is in turn determined by the type of the vehicle or the positional relationship between the antenna element 1 and the receiver set 5.
In the past, a .mu. tuning system was used exclusively as the tuning system for receiver sets. Recently, however, so-called "electronic" tuning systems have begun to be employed.
In the AM band, the conditions for coupling the antenna element 1 and the receiver set 5 differ according to the tuning system which is used. In the case of a .mu. tuning system, the electrostatic capacitance viewed from the plug of the feeder cable 4 is set at about 80 pF. In order to adjust the system to this electronic capacitance, a capacitor is usually inserted in series in the feeder cable 4. In the case of an electronic tuning system, there is no stipulation arising from the electrostatic capacitance and the sensitivity is increased when the electrostatic capacitance viewed from the plug of the feeder cable 4 is minimal. In the FM band, it is considered desirable that the impedance of the antenna be in the range of approximately 75 to 150 ohms, regardless of the type of tuning system.
The following problems have been found in conventional automobile antennas. Since both AM and FM waves are received by a single antenna element 1, a .lambda./4 grounded type antenna element (i.e. .lambda./4 with respect to the wavelength of the FM waves) is used for the antenna element 1. Specifically, for antennas for Japanese domestic use, an antenna element with a total length of 0.95 to 1.0 m is used, while in antennas for American or European use, an antenna element with a total length of 0.75 to 0.8 m is used. Accordingly, the antenna impedance ZA with respect to FM waves in this case is approximately 75 ohms.
Since an impedance matching system is used for FM waves, it is necessary to use a cable whose characteristic impedance is equal to ZA for the feeder cable 4 in order to insure matching of the antenna impedance ZA and the impedance ZC at the input terminal of the receiver set 5. In other words, it is necessary to use a coaxial feeder cable such as "3 C-2 V" cable or "2.5-2 V" cable.
However, the electrostatic capacitance of such coaxial feeder cables is approximately 67 pF/m. Accordingly, the attenuation of AM waves is extremely large, so that AM reception sensitivity drops.
In other words, when the antenna element 1 set at one of the above mentioned lengths, it is extremely short for AM waves, and thus becomes a high-impedance (capacitive) antenna. As a result, the voltage arising in the antenna element 1 is divided by the electrostatic capacitance CA and CB which are present between the antenna element 1 and the ground as shown in the equivalent circuit diagram in FIG. 11. Accordingly, the AM attenuation GA (-dB) can be expressed by the following equation: EQU GA=20 log.sub.10 {CA/(CA+CB)}
Here, CA is the electrostatic capacitance between the antenna and ground, and CB is the sum of the electrostatic capacitance CN of the attachment parts and the electrostatic capacitance CC of the feeder cable.
In the above equation, CA is determined by the length of the antenna element, and may be viewed as fixed. Accordingly, in order to reduce the attenuation GA, it is necessary to minimize CB. In this case, CN is a structurally determined fixed value; accordingly, it is necessary to minimize CC.
If D1 is the external diameter of the core conductor 41 of the coaxial feeder cable 4 and D2 is the internal diameter of the outer conductor 42 of the feeder cable 4, as shown in FIG. 12, then the electrostatic capacitance C per unit length of the cable 4 can be expressed by the following equation: EQU C=2.pi..multidot..epsilon.0.multidot..epsilon.s/log e(D2/D1)
Accordingly, the electrostatic capacitance C can be reduced by increasing the ratio of D2 to D1. However, since the characteristic impedance of the coaxial feeder cable 4 can be expressed by the equation shown below, the characteristic impedance increases when the ratio is increased. ##EQU1##
Accordingly, if such a coaxial feeder cable 4 is connected to the antenna element 2, impedance mismatching will occur with respect to FM waves, so that the FM reception sensitivity drops.
Thus, in the past, there has been no special technical means for receiving both AM and FM waves with good sensitivity. In most cases, the sensitivity for one or the other has been poor. Furthermore, a drop in the reception sensitivity of antenna systems has been a particular problem in recent years. The reason for drops in sensitivity is because since in recent years various types of electrical products which are now widely used, noise levels have increased. As a result, there has been deterioration in S/N ratios. This sensitivity drop has created various problems, such as a serious reduction or limitation of listening ranges for radio listeners.
Meanwhile, there has been a demand for smaller size and lighter weight automobile antennas. For example, there has been a strong demand for rod antennas to be shorter so as to prevent them from being damaged by obstacles such as the branches of trees, etc. Such demands generally lead to a drop in reception sensitivity. However, decreases in current levels of reception sensitivity are not acceptable and the increased cost of meeting the current demands as described above would also not be readily tolerated.